Influence of Interface Roughness and Hygrothermal Environment on the Flexural Performance of Prestressed CFRP-Strengthened Cracked Steel Beams

To meet the strengthening requirements of damaged steel beams in hygrothermal environments, this study conducted four-point bending tests on nine pre-cracked steel beam specimens. The coupled effects of surface roughness, end anchorage, prestressing level of carbon fiber-reinforced polymer (CFRP), and hygrothermal aging on the flexural behavior of the strengthened beams were systematically investigated. Results show that high-grade sandblasting (Sa3) significantly enhances interfacial bond strength through a synergistic “mechanical interlock-adhesion” mechanism, increasing the cracking load of the adhesive layer by 8.2–16.8% compared with Sa2, while partially mitigating the performance degradation caused by hygrothermal aging. The use of end anchorages effectively suppresses CFRP debonding at the beam ends, improving the ultimate load capacity and deformation performance. When a prestress equivalent to 25% of the CFRP’s ultimate tensile strength was applied, the load capacity of the strengthened beams further increased by 10.5–19.3%, interfacial cracking was effectively delayed, and the CFRP utilization efficiency reached 96.8–98.5%. Although hygrothermal exposure accelerated interfacial deterioration and reduced the interfacial cracking load, its influence on the ultimate load was relatively limited. These results offer valuable scientific and engineering insights for the design and interface treatment of CFRP-strengthened steel bridges in hygrothermal regions.